Heart failure may occur from a variety of causes including ischemic heart disease, toxins, pressure or volume overload. Recovery of cardiac function is hindered by a long known observation that cardiac myocytes do not divide in appreciable numbers during adult life. Physiologic demands for increased cardiac output are met by hypertrophy of existing cardiac myocytes through the formation of additional sarcomeres (the unitary contractile apparatus) within these cells. At the present time, the only remedy for end stage heart failure is cardiac transplant, which is limited by the supply of matched hearts and complicated by the need to suppress immune rejection. We have discovered a previously unknown subpopulation of stem cells in adult murine skeletal muscle that can be transformed into beating cardiomyocytes under primary tissue culture conditions. These cells are not satellite cells, myofibroblasts or myoblasts. A portion of the freshly isolated stem cells, injected into the vein of a mouse with chronic heart failure, will home to the heart and progress along a pathway to cardiac cell differentiation. [unreadable] We have raised a monoclonal antibody to a cell surface antigen unique to another subset of these stem cells. This novel monoclonal in combination with another antibody (Sca 1), defines 3 populations from adult skeletal muscle, which, at the time of isolation are already committed to become either: (1) adipose, (2) neuronal, or (3) cardiac cells, when grown in culture under the same standard conditions. The novel antibody defines an antigen that is present on neuronal stem cells across species and which first appears in the mouse embryo on day 12.5, the first day that neurons can be observed. When the antibody is used to harvest cells from total day 12.5 mouse embryos, the harvested cells grow into neurons in cell culture containing only FGF as a growth factor. Thus, the antigen recognized by the monoclonal is an embryonic antigen present on neuronal stem cells across species. Our previous attempts to identify this antigen are not definitive but we have also found it in yeast cells which will allow us to purify it more easily.[unreadable] Prompted by the frequency of inappropriate expression of fetal antigens in injured or cancerous tissue, we have determined that this antigen appears in a subset of brain, breast and kidney cancer. We have identified many lines in which the antigen is trapped within the cell in an internal structure called an "exocyst" This perinuclear structure is mostly unknown to cell biologists with the exception of those who study yeast. The exocyst sits next to Golgi bodies outside the nucleus and is responsible for processing proteins and membrane delived to the cell surface. We intend to continue to explore its role in both stem cells and cancer over the next year.